Mass-Movement Processes in Fine-Grained, Hemipelagic Basin: Santa Barbara Basin, California Borderland: ABSTRACT

Abstract

Mass-movement features defined by high-resolution and air gun seismic profiling are located at nine specific sites in silty clays and clayey silts on slope and slope apron settings. Two basic scales of processes occur: eight small-scale mud flows and slumps 2 to 8 m thick and 4 to 15 sq km in area extent; and a large compound feature about 100 sq km in areal extent and 2 to 25 m thick in the northeast part of the central basin. Mud flow and slump deposits sampled with box and piston cores have liquid limits (% dry basis) of 50 to 120, plasticity indices of 15 to 60, and water contents (wwb) of 40 to 65%. The compound feature has liquid limits of 75 to 105, plasticity indices of 25 to 45 and water contents of 55 to 85%. Envelopes of values of plasticity indices versus liqu d limits for different mud flows, slumps, and the compound feature show good separation, perhaps indicative of varying sedimentation rates. Mud flows and slumps exhibit a swirled x-radiography signature, fluid escape features, inclined and folded layers, and dramatic matrix-supported random fabrics with round clasts as large as 4 cm. Distorted laminations, minor faults, small folds, and homogenous sections typify different areas of the large compound feature. Suspended sediment transport, centered on the compound mass movement feature, provides faster sedimentation rates which may partly explain the feature. Only a relatively small part of the basin, the deep flat area which slopes gradually to the south, is somewhat immune to mass movement. End_of_Article - Last_Page 1001------------